Streptonigrin toxicity in Escherichia coli: oxygen dependence and the role of the intracellular oxidation–reduction state

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Abstract
The bacterial physiology of streptonigrin toxicity was further investigated. An optimal oxygen concentration for toxicity was inferred from data showing that steptonigrin at 5 µg/mL was rapidly lethal to aerobic cultures of Escherichia coli K12 JF361, but was without effect on anaerobic cultures and was bacteriostatic to cultures incubated in 5 atm of oxygen plus 1 atm of air (5 atm O2 plus air) (1 atm = 101.325 kPa). Escherichia coli were protected from a potentially lethal concentration of streptonigrin during anaerobic incubation, whether previously grown anaerobically, aerobically, or in 5 atm O2 plus air. Superoxide dismutase activity increased with increasing oxygen tension in the medium, but was not significantly changed by a lethal concentration of streptonigrin. Although the superoxide dismutase activity was four times greater in E. coli grown in 5 atm O2 plus air than those grown in air alone, the aerobic survival in 5 µg/mL streptonigrin was identical, which suggested that superoxide dismutase was not rate limiting for toxicity. Escherichia coli K12 strains deficient in glutathione (KMBL54-129, AB1157-821, and AB1157-830) were protected from streptonigrin poisoning. Dithiothreotol (5.0 mM), diamide (1 mM), methyl viologen (1 mM), and cyanide (10 mM) protected aerobic E. coli from 5 µg/mL streptonigrin.These data are also consistent with a model of in vivo streptonigrin toxicity that requires a favorable intracellular oxidation–reduction state and an optimal concentration of molecular oxygen.